FA equipment that controls FA devices is comprised of a plurality of units including a controller unit that performs arithmetic operations on the basis of a control program, an input unit that is connected to pieces of input equipment, such as an external switch and an external sensor, and that receives ON/OFF signals of those pieces of equipment as input signals, and an output unit that is connected to pieces of output equipment, such as an actuator and a relay, and that outputs output signals to them. Input/output units have memory data therewithin in order to manage their respective input/output signal data, and variables for identifying the input/output signal data held in these memory data are referred to as devices. A control operation is performed while data are transmitted and received among these units via a system bus and/or a field network in each of user program execution cycles.
In an input/output unit, an input/output response between different input/output stations (referred to as I/O stations from here on) is implemented by performing communications between the I/O stations. As an access method for use in this input/output response control, for example, there is a token passing ring access method. According to this method, transmission lines each connecting between I/O stations are connected in a ring shape, special data showing a transmission right which is referred to as a token is transmitted onto the transmission line and to the I/O stations while it is made to circulate around the ring in one direction, and only the I/O station having the transmission right performs transmission.
The time which has elapsed until the next transmission right is acquired and data transmission is performed after an I/O station has transmitted data in communications between I/O stations is referred to as the communication cycle time.
Further, a distributed input/output response control technique of, for the purpose of providing an improvement in the speed of an input/output response, making an I/O station itself perform input/output response control in an autonomous distributed way without having to use control by a management controller is implemented. Although even an input/output response which is closed within an I/O station has been conventionally controlled via a management controller, the management and control of the input/output response by the I/O station itself can provide a response which is speeded up by such an extent that the control is not performed via the management controller. Because also in an input/output response between I/O stations, control is performed without performing this control via a management controller, the communication cycle time can be shortened by such an extent and hence the response can be speeded up. Further, each I/O station has an arithmetic operation function, and can perform, for example, an AND operation, an OR operation, a register comparison operation, or the like on a plurality of input devices. Each of these input/output arithmetic operations can be implemented by performing communications also between different I/O stations.
In the distributed input/output response control, the communications between I/O stations can be grouped. According to this grouping method, I/O stations having an input/output response relation are registered as a group, and control is performed in such a way that a token virtually circulates around only within each group. Because through the grouping, the I/O network can be can divided virtually and the number of I/O stations within which one token circulates around can be reduced, the communication cycle time can be shortened.
For example, patent reference 1 discloses a method of paying an attention to the control cycle of each I/O station in order to implement a high speed input/output response in these input/outputs response control operations. In accordance with the method disclosed in this patent reference 1, each I/O station adjusts the cycle of acquiring the right of data transmission according to the value of the control cycle thereof, and an I/O station that performs control in a high-speed cycle acquires a transmission right frequently, thereby reducing the communication cycle time of an I/O station which needs a high speed response to a short one.